1. Field of the Invention
The present invention relates to a semiconductor memory device, and particularly to a semiconductor memory device with a dielectric film which is constructed from a ferroelectric thin film material or a high-dielectric thin film material.
2. Description of the Related Art
Recently, much study has been made of semiconductor memory devices which use high-dielectric thin films with greater dielectric constants than silicon oxide films, and semiconductor memory devices which use ferroelectric thin films with spontaneous polarization. High-dielectric materials and ferroelectric materials are generally oxides; more specifically, high-dielectric materials include STO (SrTiO.sub.3, strontium titanate), BSTO ((Ba, Sr)TiO.sub.3, barium strontium titanate), etc., and ferroelectric materials include PZT (Pb(Zr, Ti)O.sub.3, lead zirconate titanate), PbTiO.sub.3 (lead titanate), BaTiO.sub.3 (barium titanate), PLZT ((Pb, La)(Zr, Ti)O.sub.3, lead lanthanum zirconate titanate), of which PZT is presently under intense investigation as the most promising material for non-volatile memory devices.
FIG. 4 is a sectional view of an example of the prior art semiconductor memory devices. The semiconductor memory devices of the prior art, which use a high-dielectric material or ferroelectric material as the dielectric film of the capacitor, have a stacked structure which comprises, for example, a dielectric capacitor consisting of a lower electrode 28, a dielectric film 29 and an upper electrode 30 and is formed on a pass transistor consisting of a gate electrode 23 and a source/drain region 24, with a reduced memory cell region which serves to accomplish high integration, as illustrated in FIG. 4. In order to implement such a stacked structure, it is necessary to form a plug-structured wire 26 connecting the pass transistor and the dielectric capacitor. In FIG. 4, 21 denotes a semiconductor substrate (e.g., n-type silicon substrate), 22 denotes a LOCOS oxide film for element isolation, 25 and 31 denote layer insulator films and 32 denotes an electrode.
At present polysilicon and tungsten are widely used as plug materials for minute contact holes. Tungsten plugs, however, have the drawback that they tend to cause contact failures in drain regions during thermal processing because of their great localized stress, and are readily oxidized and sublimate in an oxidizable atmosphere, and therefore polysilicon plugs are preferably used. In addition, platinum is used as the lower electrode material for the dielectric capacitor for its excellent oxidation resistance and low reactivity.
Processes for the formation of high-dielectric films or ferroelectric films for dielectric capacitors inevitably involve their processing in a high-temperature oxidizing atmosphere at 500.degree. C.-700.degree. C. to cause their crystallization for high dielectric constants or ferroelectricity. Problems which arise when they are used to fabricate highly integrated memory devices include reaction of the lower platinum electrode of the capacitor and the polysilicon plug or the tungsten plug due to the high temperature during the process for the fabrication of dielectric films, occurrence of contact failures due to oxidation of the plug, deterioration of the transistor characteristics due to diffusion of platinum, lead from the dielectric film, etc. For these reasons, it becomes necessary to interpose a conductive, diffusion barrier layer which is thermally stable and is a large barrier against oxygen, platinum, lead and the like, between the plug and the lower platinum electrode.
As an example, ITO film, Ti film or TiN film is used as the diffusion barrier layer in Japanese Unexamined Patent Application Disclosure HEI 4-85878. The ITO film deteriorates at 500.degree. C. or higher temperatures, and particularly its resistance increases at 600.degree. C. or the sintering temperature of the ferroelectric substance, while the Ti film and TiN film, when subjected to oxygen annealing at 600.degree. C., undergo oxidation of the diffusion barrier layer which causes deterioration of its conductivity and delamination thereof.
On the other hand, use of titanium, titanium nitride film or the like as the diffusion barrier layer presents a problem that oxygen, lead, platinum, etc. tend to diffuse through the grain boundary due to their columnar crystal structure, and the film thickness is required to be 2000 A or greater in order to obtain satisfactory barrier characteristics. The increased film thickness naturally causes increase in the step height of the capacitor region, thus preventing high integration and bringing a serious obstacle against their practical use.